Recently the company I work for acquired another business which had over 1 million users at one point. I've been having trouble finding an email service that I feel will be able to handle this heavy of a load. So far I've looked into SendGrid and MailChimp but I haven't been blown away by either.
- Hey Josh. All of the big transactional/campaign email services will be able to handle that volume. Make sure you contact them after signing up and let them know. Mail providers tend to shut down high volume accounts quickly if you don't have a track record of sending email with them (due to how much people use their services for abuse). It's going to be the ease of sending/deliverability that will make the biggest difference in your case. If this is a 1 time email. Personally, I'd avoid transactional and would send it as a campaign. Just because any transactional mails require custom development. All of the "campaign" providers allow you to design in a WYSIWIG interface & send. Could break your list into multiple parts and send multiple campaigns throughout the month. For best deliverability, I'd first look at using the other companies domain / whatever service they used to send email. Because there is a track record of emails from that address & the users are familiar with the name. If people see email coming from something they don't recognize, high possibility of them marking as spam. At PH we use Mailjet. They're great & handle both Transactional/Campaign. But in your case, I'd try to use whatever provider they were already using to send mail.